SATURDAY, Jan. 5, 2019 (HealthDay News) -- After suffering through a cold, many people still have a persistent cough -- but why?

According to Dr. Jonathan Parsons, director of the Asthma Center at Ohio State University Wexner Medical Center, here are some reasons for a continuing cough:

Coughing is protective. It removes irritants from the lungs and protects the airway.

The cough might be due to an upper respiratory tract infection. Even after the infection is gone, you may still have some inflammation in the lungs.

Coughing can be worse at night. "For some patients, their cough is so severe that it disrupts their sleep to the point where they aren't getting any sleep at all, which impacts their ability to function during the day. In that situation, you might consider taking a cough suppressant with codeine to blunt the cough reflex and assist with sleep," Parsons said in a center news release.

A cough that lasts more than three weeks may be a chronic condition. If you're still coughing and feeling sick after three weeks, then you should see a doctor.

Causes of a chronic cough include:

Uncontrolled allergies,

Uncontrolled asthma,

Side effects of medicine,

Acid reflux.

Persistent cough might be a symptom of a serious illness.

"If you're coughing up blood, spiking fevers or have significant shortness of breath associated with the cough, you need to be evaluated quickly. You could have walking pneumonia. If you're a smoker, it could be cancer. A doctor will examine you to determine the cause of the cough and establish a treatment plan," Parsons said.